OBJECTIVE This study was designed to identify factors that predict longitudinal increases in corneal curvature as measured by the First Definite Apical Clearance Lens (FDACL) and flatter keratometric reading (Flat K) in keratoconus. METHODS The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a long-te rm evaluation of the natural history of keratoconus involving 1209 patients. This report uses 8 years of follow-up data from 1032 patients who had penetrating keratoplasty in neither eye at baseline and who provided enough data to compute the slope of the change with time in the FDACL or the Flat K. Outcome measures included the aforementioned slopes and whether the FDACL or the Flat K increased by 3.00 or more diopters (D) in at least 1 eye. RESULTS At CLEK Study entry, patients were aged 38.9 +/- 10.8 years. Overall, 44.3% of them were women, and 69.3% of them were white. The slope of the change in FDACL (0.18 +/- 0.60 D/y) and Flat K (0.20 +/- 0.80 D) during 8 years translates into expected 8-year increases of 1.44 D in FDACL and 1.60 D in Flat K. Increases of > or = 3.00 D in either eye had an 8-year incidence of 24.8% for FDACL and 24.1% for Flat K. Independent predictors of increased FDACL included younger age, poorer baseline high-contrast manifest refraction visual acuity, and non-white race. Younger age and poorer high-contrast manifest refraction visual acuity were independent predictors of a >3.00-D increase for both FDACL and Flat K. CONCLUSIONS CLEK patients exhibited a slow but clear increase in corneal curvature. Younger age and poorer high-contrast manifest refraction visual acuity at baseline predicted the rate of change in corneal curvature.